Not applicable.
Not applicable.
The invention relates generally to a light polarization device, such as are commonly used in projection display systems, and more particularly to a light integrator that provides an enhanced polarized output.
A light integrator, such as a rod or light tunnel, is commonly used for homogenizing the output of an illumination source in projection display systems to provide uniform illumination to a spatial light modulator(s) (xe2x80x9cSLMxe2x80x9d), but may be used in other applications where it is desired to light from a relatively small source into a larger field of uniform illumination. In some projection display systems, liquid crystal spatial light modulators are used. It is often desirable to illuminate such modulators with polarized light. These modulators rotate the incident polarized light when switched between the xe2x80x9conxe2x80x9d and the xe2x80x9coffxe2x80x9d states.
A light pipe may be a bar or rod of glass, often rectangular, that typically has a relatively small aperture at one end that receives light from a lamp system. The lamp typically has a small arc about 0.7-1.3 mm that generates the light, which is focused onto the aperture with mirrors and lenses. Light emitted by the illumination source enters the aperture on the input face of the light pipe and is reflected off the walls of the light pipe until it is transmitted out the exit face. The light enters the aperture such that the angle of incidence with the wall(s) of the light pipe result in essentially all of the light being reflected back into the light pipe. This type of reflection is commonly referred to as xe2x80x9ctotal internal reflectionxe2x80x9d (xe2x80x9cTIRxe2x80x9d). The exit face is frequently shaped to conform to the SLM to provide light that is spatially uniform in intensity.
A light tunnel is another type of light pipe, but rather than being a solid bar, a reflective xe2x80x9cboxxe2x80x9d is formed using metallic mirrors or other reflectors. As with a light rod, light entering the light tunnel is reflected off the walls and is transmitted out the exit face. Since TIR is not relied upon with the mirrored light tunnel, the angle of incidence with the wall of the light tunnel is not as critical. Also, since the light tunnel is typically filled with air or other gas(es), transmission loss through the light tunnel can be relatively low, and other optical effects that might arise in a glass rod, such as birefringence, can be avoided.
It is generally desirable to provide a bright display that is efficient and reliable. Unfortunately, conventional absorptive polarizers can cut the light output essentially in half, depending on the characteristics of the polarizer. Various techniques have been developed to mitigate this loss. One approach is to separate s and p polarized light externally by segregation into focal spots at a virtual image plane. An external polarization rotator rotates the offset focal spots 90 degrees. The light pipe separates the s and p polarization using TIR with a double layer wall of the light pipe that is parallel to the optic axis of the light pipe. The interface between the walls has either a polarization-separating film or thin films with optical birefringence such that the s polarized light undergoes TIR at the inner wall and p polarized light undergoes TIR at the outer wall. An additional optical element deployed externally to the light pipe provides recombination of the focal spots into a rectangular field for illuminating a liquid crystal image modulator.
A reflective polarizer is coupled to the output of a light integrator to recover non-transmitted light. Light is reflected off the polarizer and re-enters the light integrator, which includes a reflector at a portion of its input face. The light traverses the light integrator from the exit face to the input face and back again through TIR, in the case of a light rod. This re-entrant light is shifted in phase by the reflections before exiting the light integrator. A portion of this phase shifted light is transmitted by the polarizer, thus the light is recaptured.
In one embodiment, phase shifting occurs primarily through reflection, that is, no additional phase shifting elements are required in the system. The light pipe could be made relatively long, increasing the number of xe2x80x9cbouncesxe2x80x9d (reflections) in a round trip between the exit and input faces. Other factors being constant, a longer light pipe would achieve more rotation per round trip at the expense of absorption loss or other loss mechanisms. Alternatively, the light pipe could be relatively short. A short light pipe would not have as many bounces per round trip, but the recovered light could make many round trips. Some of the recovered light is lost out the aperture at the input face, thus it is generally desirable to make the aperture small. Comer reflectors can be added to increase the number of reflections per xe2x80x9ctripxe2x80x9d without significantly increasing the transmission length.
In another embodiment, a polarization state modifier, such as a circular or form birefringent element or other optically active element, phase-shifting coating, or retarder plate, is placed in the light path between the exit face and the input. A retarder plate can be essentially normal to the optic axis of the light pipe, or off-normal. In a particular embodiment, a phase-shifting coating is applied to the walls of the light pipe to enhance the phase shift occurring with each TIR bounce.